t 608 
35 U6 
spy 1 



U. S. DEPARTMENT OF AGRICULTURE, 
BUREAU OF ENTOMOLOGY— BULLETIN No. 99, Part I. 

L. O. HOWARD, Entomologist and Chief of Bureau. 



PAPERS ON INSECTS INJURIOUS TO CITRUS 
AND OTHER SUBTROPICAL FRUITS. 



THE ORANGE THRIPS: 

A REPORT OF PROGRESS FOR THE 
YEARS 1909 AND 1910. 



P. R. JONES and J. R. HORTON, 

Agents and Experts, Deciduous Fruit Insect Investigations. 



Issi i;d March 6, 1911. 




WASHINGTON: 

GOVERNMENT PRINTING OFFICE, 
1911. 



7 






ib-series 



ILLUSTRATIONS 



Page. 
Plate I. Fig. 1. — Young oranges showing injury by the orange thrips (Euthrips 
titri). Fig. 2. — Young oranges showing injury to stem and blos- 
som ends by the orange thrips 4 

II. Mature oranges showing injury due to the orange thrips 4 

III. Orange foliage showing curled and distorted condition of leaves due 

to work of the orange thrips 4 

TEXT FIGURES. 

Fig. 1. — Diagram illustrating the relative abundance of orange thrips on oranges, 

on orange foliage, and on other plants during the season 7 

2. — Power spraying outfit in use in spraying for the orange thrips 14 

IV 



U. S. D. A., B. E. Bui. 90, Tart I. D. F. I. I., March 6, 1911. 

PAPERS ON INSECTS INJURIOUS TO CITRUS AND OTHER SUBTROPICAL 

FRUITS. 



THE ORANGE THRIPS: A REPORT OF PROGRESS FOR 
THE YEARS 1909 AND 1910. 

By P. R. Jones and J. R. Horton," 
Agents and Experts, Deciduous Fruit Insect Investigations. 

INTRODUCTION. 

The orange thrips (Euthrips citri Moulton), a small, yellow, active 
insect belonging- to the order Thysanoptera (popularly known as 
thrips) , scars the fruit and curls and distorts the leaves of the orange. 
At the present time its control constitutes the chief insect prob- 
lem confronting the citrus growers of the San Joaquin Valley 
orange belt of California, which winds along the Sierra Nevada foot- 
hills, from east of Fresno to south of Delano. This insect, the work 
of which was first noticed 15 or 16 years ago, has increased in num- 
bers with the growth of the citrus industry and recently has assumed 
serious economic importance. 

At the urgent request of a number of orange growers of Tulare 
County, an investigation of the insect was begun the latter part of 
April, 1909. The present paper is a preliminary report of the results 
obtained during the seasons 1909 and 1910. 

The writers wish to acknowledge the financial assistance of the 
Tulare County board of supervisors, the Lindsay Citrus Growers' 
Protective League, and the Tulare County Fruit Exchange; they 
desire to acknowledge the kindness of Messrs. P. M. Baier, Harry 
Postlethwaite, and R. H. Shoemaker in allowing the Bureau of Ento- 

°The investigation of the orange thrips by members of the force engaged in 
studies of deciduous-fruit insects appeared desirable, because these men were 
familiar with a closely related species — the pear thrips — which is very destruc- 
tive to prunes, pears, cherries, etc., in the San Francisco Bay region. However, 
in order to keep together the articles dealing with insects damaging citrus and 
other subtropical fruits, the present paper is published in a series of articles 
dealing with insects of that class. — A. L. Quaintance, in Charge of Deciduous 
Fruit Insect Investigations. 

1 



2 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. 

mology the use of their orchards for experimental and demonstration 
purposes; and they would express their indebtedness to the large 
number of orange growers in Tulare County who have put into effect 
in their own orchards the recommendations of the Bureau, thereby 
demonstrating the value of the spraying treatments advised. 

ORIGINAL HOME AND DISTRIBUTION. 

The orange thrips is probably native to North America. Its natu- 
ral habitat is probabty the Sierra Nevada foothills or the adjoining 
plains of the southern San Joaquin Valley, and it was no doubt 
attracted from its natural food plants by the more succulent and 
luxuriant orange trees. This insect is distributed throughout the 
entire orange belt of the San Joaquin Valley and has been collected 
in several places in Southern California and at Phoenix, Ariz., by 
the senior author. The infestation in Arizona embraces orange 
groves in the Salt River Valley surrounding Phoenix, and was re- 
ported upon by Prof. J. Eliot Coit in a bulletin of the Arizona Agri- 
cultural Experiment Station." This gentleman, in sending specimens 
to Dr. W. E. Hinds for identification, probably did not obtain the 
true orange thrips (Euthrips citri Moulton), but some specimens of 
Euthrips occidentalis Pergande, which is found occasionally upon 
citrus trees, but which rarely causes any serious injury. The true 
orange thrips was described as a new species by Mr. Dudley Moulton 
in a bulletin of the United States Department of Agriculture, issued 
February 11, 1909. & 

The orange thrips has also been reported from Hermosillo, Sonora 
Province. Mexico, but the writers have not been able to obtain speci- 
mens from that locality. 

The occasional scarring of oranges in the north-central portion of 
California is caused by the grain thrips (Euthrips tritici Fitch), and 
not by the orange thrips. 

FOOD PLANTS. 

Although the orange thrips. when described, was thought to infest 
only citrus trees, the writers have taken it from a number of other 
host plants. The following list shows the wide range of food plants 
upon which this insect can exist : 

Of citrus fruits the following are affected: Citrus aurantivm var. 
sinensis (Washington Navel, Australian Navel (?), Thompson Im- 
proved. Valencia Late, Mediterranean Sweet, Parson Brown, Ruby 

Arizona Agricultural Experiment Station, Bulletin No. 5S, Citrus Culture in 
the Arid Southwest, p. 319, 1908. 

6 IT. S. Department of Agriculture, Bureau of Entomology, Technical Series 
No. 12, Part VII. 



THE ORANGE THRIPS. 3 

Blood. St. Michael. Homosassa, and seedlings) ; Citrus nobilis (Sat- 
suma and tangerines) ; Citrus decumana (grapefruit) ; Citrus medica 
var. limon (lemon) ; Citrus medica var. acida (lime, varieties of) ; 
and Citrus japonica (kumquat). 

The following miscellaneous plants are infested: Punica gramatum 
(pomegranate) ; Vitis vinifera (European grape, varieties of) ; 
Schinus molle (California pepper tree); "umbrella tree;" Pyrus 
communis (pear); Primus armeniaca (apricot); Primus persica 
(peach) ; Prunus domestica (European plum, varieties of) ; Salix sp. 
(willow) ; Rumex sp. (dock) ; Portulaca oleracea (purslane) ; Olea 
europi a (olive) ; Rubus idceus (red raspberry) ; Rosa sp. (rose) ; 
Solatium sp. 

CHARACTER AND EXTENT OF INJURY. 

Injury to citrus trees and fruit is caused directly by the feeding 
of both adults and larva* upon the surface of the parts attacked. 
This feeding may be on the young fruit (Plate I, figs. 1,2), the nearly 
mature fruit (Plate II), or the new, tender foliage (Plate III), and 
generally takes place on all of these. The injury to foliage is gen- 
erally on young leaves, but may also occur on the axillary buds. 

The manner of feeding of both the adult and larva of the thrips is 
identical, and consists in piercing the plant tissues with the sharp 
mouthparts with which both stages are equipped and then rasping 
the wound by a " rooting " motion of the head. The vegetable juices 
thus liberated from the plant cells are sucked into the alimentary 
canal of the insect. The characteristic marking or scabbing of the 
fruit, so noticeable at picking time, is started when the fruit is very 
small— just after the petals have fallen from the blossoms. This 
scabbed area is small at first, but as the fruit grows and the thrips 
continue to feed the markings deepen and at the same time the area 
of injury is enlarged. The continued feeding of a large number of 
thrips results in the scabbing of nearly the entire surface of the fruit. 
Often the marking is so large and deep over a portion of the orange 
that it causes the fruit to be misshapen and aborted. Frequently the 
entire surface is scarred while the fruit is still small, with the result 
that it ceases to grow and falls from the tree. 

Orange trees in the Tulare County citrus belt make about four dis- 
tinct growths a year, and it is on this tender foliage that the orange 
thrips multiply in greatest numbers. The feeding of large numbers 
of these little insects causes the young leaves to curl and become dis- 
torted and the whole growth to present a sickly appearance. Young 
trees are often held back a year or more in growth by the prompt 
destruction of the terminal buds soon after these make their 
appearance. 



4 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. 

DESCRIPTION AND LIFE HISTORY. 
THE ADULT. 

The adult female of the orange thrips is a small, four- winged, 
orange-yellow insect, which moves very rapidly by running, leaping, 
and ftying. The mouthparts, which are suctorial in nature, form a 
sharp cone projecting from the underside of the head. The adult 
male is smaller than the female and much more rapid in its 
movements. 

The original description of the adult female by Moult on a is as 
follows : 

Euthrips citri n. sp. 

Measurements: Head, length 0.75 nun., width 0.15 mm.; prothorax, length 0.09 
mm., width 0.18 mm. ; mesothorax, width 0.24 mm. ; abdomen, width 0.25 mm. ; 
total body length 0.86 mm. Antennae: I, 12/*; II, 36/*; III, 39/*; IV. 39/t ; V. 30/*; 
VI, 34 M ; VII, 6/t; VIII, 12/*; total, 0.205 mm. Color, yellow to orange-brown, 
with thorax and segment 2 of antennae more noticeably orange-brown. 

Head twice as wide as long, retracted considerably into the prothorax, 
broadly rounded in front, with only slight depressions to receive the basal 
joints of the antenna?; two spines on anterior margin, other spines not conspic- 
uous; cheeks almost straight and parallel. Eyes large, occupying almost one- 
half the length of the head, prominent: pigment deep red to purple: facets of 
eyes large, eyes pilose. Ocelli subapproximate, margined inwardly with yel- 
low-brown crescents. Mouth-cone short, reaching almost to posterior margin 
of prothorax, broadly rounded and with black spot at tip ; maxillary palpi 
3-segmented. Antennas 8-segmented, with segment 2 orange-yellow, other seg- 
ments uniformly light brown: segments 2, 4, 5, and 6 almost equal in length; 
style about one-half the length of segment (i. All spines inconspicuous; sense 
cones transparent. 

Prothorax about twice as wide as long, posterior angles broadly rounded; 
with long brown and outer small spine at each posterior angle, other spines not 
conspicuous. Mesothorax largest and with anterior angles broadly rounded. 
Legs light yellow-brown, with tarsi lighter but dark brown at the tip ; spines 
on legs brown. Wings present and fully developed, forewings broadest near 
base and pointed at tips; with the ring vein and a single longitudinal vein 
which divides at about one-third the length of the wing from the base, the 
anterior part running parallel and approximate to the anterior part of the ring 
vein, and ending abruptly near the tip, the posterior paralleling and approach- 
ing the posterior part of tbe ring vein and ending about one-half the wing's 
length from the end, each branch with a dark-brown marking immediately at 
its tip. The costa bears a row of about 29 regularly placed spines. Other 
spines placed as follows: A group of 5 near base of median longitudinal vein; 2 
on either side of where second vein branches from the first, and 3 scattered 
spines about equidistant on each branch vein and in each case one of these 
spines immediately at the end of the vein; several rather long spines on scale. 
Veins of the forewing unusually strong and conspicuous, somewhat orange 
colored near base but fading to yellow near tip. Membrane of wings trans- 
parent. 

a Loc. cit. 



Bui. 99, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. 



Plate I. 




Fig. 1.— Young Oranges, Showing Injury by the Orange Thrips (Euthrips citri). 
Somewhat Enlarged. (Original.) 




Fig. 2.— Young Oranges, Showing Injury to Stem and Blossom Ends by the 
Orange Thrips (Euthrips citri). Somewhat Enlarged. (Original.) 



Bui. 99, Part I, Bureau of Entomology, U. S. Dept of Agriculture. 



Plate II. 




Mature Oranges, Showing Injury Due to the Orange Thrips. (Original.) 



Bui. 99, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. 



Plate III. 




Orange Foliage, Showing Curled and Distorted Condition of Leaves Due to 
Work of the Orange Thrips. (Original.) 



THE ORANGE THRIPS. 5 

Abdomen ovoid, tip conical, all spines, excepting a very few at tip, incon- 
spicuous. 

Described from many female specimens collected from orange foliage and 
fruit at Exeter, Tulare County, Cal. 

The males are similar to the females, but smaller and more active, 
with the orange-colored testes prominent. 

THE EGG. 

The egg is a bluish white, bean-shaped object measuring from 0.2 
mm. in length to about O.OTo mm. in width, with a very thin shell. 

THE LARVA. 

First-stage larva. — Length 0.041 mm.; width of mesothorax 0.011 mm.; 
general shape fusiform. The antennae, head, and legs are large and unwieldy 
in proportion to the rest of the body. Color translucent white. Antenna-, length 
0.015 mm. ; distinctly 4-segmented ; I short, cylindrical ; II more than twice as 
long as I, slightly urn-shaped, longer than wide; III about as long as II. 
obtusely fusiform; IV about as long as the other joints combined, fusiform, 
very finely drawn out at the distal end. Segments II, III, IV (II very ob- 
scurely) ringed, the distal rings on segment IV appearing as segmental 
divisions. A few fine hairs present on all segments, most numerous on IV but 
not very conspicuous on any of the segments. Head subquadrate ; eyes reddish- 
brown. Abdomen gradually tapering, 10-segmented, first 8 segments subequal; 
IX and X large and more abruptly tapering, hairs inconspicuous. Legs stout, 
femora and tibia? nearly equal in length, tarsi one-jointed, ending in a single 
claw. 

Second-stage larva. — Length 0.9 mm.; head length 0.1 mm.; width 0.0S3 nun.; 
length of antennae 0.175 mm. ; width of mesothorax 0.200 nun. : widtli of 
abdomen 0.3 mm.: Antennas, I, 2.u ; II, 3/x; III, 9/*; IV, 45/*; V, 9fi; VI, 15/m; 
color orange-yellow. In shape similar to first-stage larva except that the 
abdomen is oval to ovate and generally more robust. //(•*/(/ quadrate, small in 
proportion to body, eyes reddish. Antennae apparently 4-segmented under 2/3 
objective, but under 1/6 objective distinctly 6-segmented, the chitin not extend- 
ing into the fifth and sixth segments; I short, conical, about as broad as long; 
II cylindrical, broader than long and slightly longer than I; III obtusely 
spindle-shaped, about twice as long as broad and about as long as I and II 
combined: IV obtusely spindle-shaped but blunt on the distal end, about as 
long as III ; V very short and thick, slightly broader than long, about one- 
fifth as long as IV ; VI cylindrical, longer than broad, about one-third as long 
as IV. Abdomen oval to ovate. 10-segmented, the last segment tubular. Legs 
short and stout, hiud femora and tibiae about equal, hairs everywhere incon- 
spicuous except a few under 1/6 objective, which are the most prominent on 
last segments of antennae. 

THE PUPA. 

First-stage pupa. — Length 0.56 mm.; width of head 0.15 mm.; width of 
mesothorax 0.18 nun.: width of abdomen 0.25 mm.; antenna 1 , length 0.2 mm. 
Color pale translucent yellow; antennae, legs, and wing-pads lighter. Shape 
similar to advanced first-stage larva ; abdomen elongate ovoid. Antenme pro- 
jecting cepbalad, 4-segmented; I short, thick, slightly wider than long'; II ob- 
tuse, urn-shaped, about as wide as long; III obtusely spool-shaped, about as 

7S562°— Bull. 99, pt 1—11 2 



6 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. 

long as I and II combined and about twice as long as wide; IV about as long 
as III, tapering to obtuse apex. Wing-pads extending to distal margin of the 
second abdominal segment, those of hind wings slightly longer. Legs stout, 
hind femora and tibia? about equal. Hairs present on live specimens but not 
prominent, short, slightly longer on tip of abdomen. 

Second-staye pupa. — Length 0.GG6 mm.; width of head 0.133 mm.; width of 
prothorax 0.133 mm. ; width of mesothorax 0.16G mm. ; width of abdomen 0.133 
mm. Shape similar to that of the adult. Color translucent white to pale yel- 
lowish ; eyes reddish, more prominent than in first-stage pupa. Antenna? 4-seg- 
mented, projecting backward over the head and thorax and reaching to the 
middle of the prothorax, second segment forming a kind of elbow from which 
3 or 4 long seta? project cephalad. Prothorax nearly twice as broad as long; 
wing-pads in pupa? just entering the second pupal stage extending to the 
distal margin of the sixth abdominal segment; in pupa? in which the adults 
are nearly ready to emerge the wing-pads extend to the distal margin of the 
ninth abdominal segment. Abdomen similar in shape to that of the adult. 
Legs stout, hind femora and tibia? about equal in length, seta? more prominent 
than in first-stage pupa, longer at the tip of the abdomen ; conspicuous in fresh 
specimens but not in mounted ones. Tip of abdomen often with a cremaster- 
like formation resembling in shape a fork with 4 tines. Male pupa? smaller, 
resembling the adults, their wing-pads usually reaching past the tip of the 
abdomen. Seta? usually not so prominent. 

SEASONAL HISTORY. 

The orange thrips passes the winter in the adult state, and it is 
generally the adult form which first becomes conspicuous upon the 
orange trees in the spring. Although no large number of adults 
has been collected in hibernation, these undoubtedly pass the winter 
in sheltered places, such as the dead leaves and twigs forming the 
trash under most orange trees; they are occasionally found on living 
plants and on citrus nursery stock in midwinter. 

Adult thrips appear in limited numbers during March. They 
deposit very few eggs in the early part of April, prior to the blossom- 
ing of the Navel orange trees, but soon after most of the petals have 
fallen larva? become quite numerous. Oviposition has not been 
observed, but it is probable that it takes place mostly at night. 
Examinations for eggs revealed the fact that most of them are placed 
in the new, tender growth, being inserted into both upper and lower 
leaf surfaces, and also in the shoots. They are also placed in the 
receptacles of the blossoms after the petals have fallen and in young 
fruit and fruit stems. 

The larva? are wingless and when full grown are orange colored. 
When ready to pupate they fall from the trees, get into a curled dead 
leaf, amid cobwebs, dust, and leaf particles, and hide until the trans- 
formation is completed. Pupa? are not found in numbers propor- 
tionate to the larva' and adults, since it is in this stage that the 
mortality rate of the insect is greatest. The pupa? are very soft- 
bodied and less active than larva? and adults. They move readily, 
however, when disturbed. 



THE ORANGE THRIPS. 



Eggs, larvae, and adults are found on the trees, and pupa? in the 
dead leaves under them, from early May until early November, all 
four forms being present during the entire period. The broods thus 
overlap so closely that it is very difficult to separate them. 

INTERRELATION OF ABUNDANCE OF THRIPS AND FOOD PLANTS. 

The orange thrips feed only on very tender plant tissues, namely, 
the young leaves, shoots, and tender fruit. This makes it necessary 
for them to pass from foliage to fruit and from plant to plant as the 
suitability of the tissues as food changes. They first make their 
appearance in April and May on the new growth of the Navel orange, 
reaching the first maximum of abundance about the time four-fifths 
of the petals are off'. When most of the petals have fallen a few 
thrips pass to the more advanced fruit and the number feeding on 
the latter rapidly increases as the first growth of foliage becomes 
hardened and distasteful. The thrips continue feeding on the fruit 
until the latter, in turn, becomes somewhat tough, and reach a second 



APR. 1~MAY | JUNE I JULY I AUG. I SEPT. I OCT. I NOV. 




30 6 12 /8 24 30 e 12 13 24 30 6 12 IS 24 30 6 12 18 24 30 S 12 18 24 30 6 /2 34 30 e 12 18 24 



4BC/ HDA/VCE Or. OffA(VG£ THfi lPS \ 
" OA/ OffA/VGES | \ • 



\AB[MDAA/C£ pr, OffAA/G£ 
\THWPS OA/~0#AA/G£ 'EOL/AGE. 

,aboa/oaa/ce or om/vge'^ ™x / &~? / ¥<?£?f'f/? i .I$f£.?i 

GaAP£sSvMB#£ttA~r#EES~&^/SdE^ 



i i i l i I i i 



B 



j^j_ 







v 



\ 



Fig. 1. — Diagram illustrating the relative abundance of orange thrips on oranges, on 
orange foliage, and on other plants during the season. (Original, i 

and greater maximum in May. June, and July. They then pass 
once more to the succulent growth which has come on in the mean- 
time, and reach the third and final maximum of concentration in 
August and September. 

As the first citrus growths are becoming tough and before the fruit 
is quite tender, the thrips begin to work on the leaves of the grape, 
pepper tree, umbrella tree, and some uncultivated plants, reaching 
a minor maximum of abundance on these at the time of greatest 
abundance of tender leaves and stems. A second maximum of concen- 
tration is reached on some of these secondary food plants in the fall, 
when most all of the summer growths on citrus trees have become 
tough. 

The relative abundance of the orange thrips on its various food 
plants, at different times during the season, is shown diagrammatic- 
ally in the accompanying chart (fig. 1) ; the diagram represents the 
results of observations made at regular intervals in different parts 
of the Tulare County citrus belt. 



5 INSECTS INJURIOUS TO SUBTROPICAL. FRUITS. 

LIFE CYCLE. 

In ascertaining the length of the life cycle the average lengths 
of egg, larval, and pupal stages were added together, and to this an 
additional 3 days, which was the usual time from the appearance of 
the adult female until ovipositing began. The life cycle thus in- 
cludes the period from egg to egg, or from the time the egg has left 
the abdomen of the female of one brood until the eggs of the next 
brood appear. 

Egg stage. — The length of the egg stage was determined by con- 
fining adult thrips on potted orange plants overnight, then remov- 
ing all insects and examining the plants twice daily, and counting 
the larvae hatched until they cease to appear. The length of the 
egg stage of 19 eggs during the month of August, 1909, was found 
to be 2i days for a minimum and 8 days for a maximum, with an 
average of 6.2 days. Eggs deposited the latter part of September 
required from 20 to 24 days for incubation. During May. June. 
July, and August, 1910. observations on 15 eggs gave a minimum of 
5 days, a maximum of 13 daj^s. and an average of 8.1 days for 3 
months. It is probable that the majority of eggs deposited during 
May, June, July, and August would require from to 8 days for 
incubation, while in March. April. September, and October the 
length of the egg stage would be considerably more. 

Larval stage. — The number of days required for the development 
of the larva varied from a minimum of 3 days to a maximum of 13 
days, with an average of G.OG days for 55 individuals: and a mini- 
mum of 3 days, a maximum of 13 days, and an average of 7.2 days 
for 73 individuals during April, May. June, July, and August. The 
length of the larval stage would probably be extended, similar to the 
egg stage, during September and October. Two distinct larval 
stages were observed. The first stage is usually about two-thirds as 
long as the second, and the larva? more active. 

Pupal stage. — The pupal stage Avas best observed by keeping larva? 
in confinement until the}^ pupated. The total length of the pupal 
instar for 30 individuals, under observation during June and July, 
1909, varied from 2 to 5 days, with an average of 3.0 days; while 
287 observations during April to August. 1910. gave a variation of 
2 to 7 days, with an average of 1.8 days. Two pupal stages were 
observed, there being a distinct molt from the first to the second 
stage, which begins with a splitting of the skin from the head back 
dorsally to about 7 to 9 abdominal segments. The pupae are more 
active in the first than in the second stage. 

Total life cycle. — The life cycle, obtained by adding the aver- 
age lengths of egg, larval, and pupal stages, and allowing 3 days 



THE ORANGE THRIPS. 9 

before eggs were deposited by the newly formed adults, made a 
total of 18.G8 days for May to August, inclusive, 1909. For the 
months April to August, inclusive, 1910, this period was 23 days. 
The length of the life cycle of 8 individuals actually recorded from 
the egg, upon potted plants, allowing 3 days, as before, for the 
adults to oviposit, varied from 20 to 36 days. The data upon the 8 
individuals was obtained during September and October, and the life 
cycle was undoubtedly longer at this time than in midsummer. The 
length of life of the adults observed on confined individuals was 
from 4 to 36 days. 

X umber of broods. — Although the number of generations in a sea- 
son has not been definitely observed, there are probably four and a 
partial fifth during the period of May to July, inclusive, and one 
generation in each of the months March, April, August, September, 
and October, making in all a possibility of eight to ten generations 
for the season. 

HABITS. 

The orange thrips is very active, especially in the adult form. Its 
ability to run. leap, and fly is much greater than that of any other 
thrips so far observed by the writers. This activity and their 
small size allow them easily to pass unobserved. The writers have 
frequently seen adults fly from one tree to another 20 feet or more 
distant. They undoubtedly move about to a certain extent, and will 
go from one orchard to another in search of suitable food. Fre- 
quently they will desert the orange groves, between periodical 
growths, for grapes and certain deciduous fruits. 

The orange thrips appear to thrive best in sunny and even very 
hot weather. On cool cloudy days they are less active and generally 
group themselves on the underside of the leaves. 

Their reproductive habits are only partially understood. Males 
are present part of the year, but usually in more limited numbers 
than the females. 

EXPERIMENTS WITH METHODS OF CONTROL. 

CULTIVATION. 

Attempts have been made to control the orange thrips, in part, by 
means of cultivation, but none of these endeavors has been in the 
least successful. One orchard was hand-raked under the trees and 
the soil stirred up in the fall, with the hope that pupse would be de- 
stroyed, but results were negative. Another orchard which was 
plowed deeply in the fall yielded similar results. 



10 



INSECTS INJURIOUS TO SUBTROPICAL, FRUITS. 



FUMIGATION. 

Some experiments have been conducted in the hope that fumiga- 
tion with hydrocyanic-acid gas would prove effective in controlling 
the orange thrips. but all results have been unsatisfactory, because 
of the activity of the insects, the large number of generations, and the 
expense of the operation. 

SPRAYING. 

The only method of control which has given good results is spray- 
ing at high pressure with a contact insecticide. No sprays aside from 
those which kill by contact have been tried because such sprays have 
been unsuccessful in controlling other species of injurious thrips. 

Experiments to Determine Killing Effect of Different Sprays. 

The following spra} 7 s were tested in the field for killing effect on 
the thrips: Homemade distillate-oil emulsion, in combination with 
black-leaf tobacco extract, which is a dark, almost viscid liquid con- 
taining 2f per cent nicotine; and commercial lime-sulphur (33° 
Baume) in combination with the tobacco extract. All sprays were 
applied with a hand pump, maintaining a pressure of 140 pounds. 
A large number of young fruit was examined for live and dead 
thrips. While this method did not give absolutely accurate results, 
because of the number of thrips knocked off by the force of the 
spray, it offered some means of comparison. Table I shows the 
relative killing effect of the various washes : 

Table I. — Killing effect of various sprays on orange thrips. 



Number of fruits 
examined. 



150 

200 

100 

Several hundred 

Do 

Do 



Blackleaf 1-50 and distillate-oil emulsion 1 per cent . . 
Blackleaf 1-tiO and distillate-oil emulsion 1 per cent. . 
Blackleaf 1-80 and distillate-oil emulsion 1 per cent. . 
Blackleaf 1-85 and distillate-oil emulsion 1 per cent. 

Commercial lime sulphur 1-75 and blackleaf 1-50 

Commercial lime-sulphur 1-50 and blackleaf 1-100. . . 



Total 
number 
of thrips 
counted. 



129 
182 
67 



Number 

of thrips 

dead. 



126 
170 
64 



Percent- 
age of 
thrips 
dead. 



97.6 
93.4 
'92.5 
75 
90 
95 



Experiments to Prevent Marking of the Fruit. 



Experiment Xo. I. — A block of 150 Washington Xavel orange 
trees was sprayed three times with distillate-oil emulsion and black- 
leaf tobacco extract; the former at the strength of 2 per cent and 
the latter in the proportions of 1 to 80 and 1 to 100 parts of spray. 
The spraying was tried as a means of preventing the thrips from 
curling the tender foliage and marking the young fruit. The first 
application was made May 4, 1909, after most of the petals had fallen 



THE ORANGE THRIPS. 



11 



and when both larva 1 and adults were present. The second applica- 
tion was made eight days later, and the third three weeks after the 
second, at which time the thrips began again to be numerous. All 
the spraying was done with a hand outfit, maintaining a pressure of 
140 pounds. 

In recording the results of the spray applications to ascertain their 
efficiency it was necessary to class the fruit, as regards injury, in four 
grades, as follows : 

Sound: No thrips marking. 

Slightly marked : A slight marking at one end or a few streaks on the surface. 

Moderately marked : Both ends of fruit marked and some scabbing on the rest 
of the surface. 

Badly marked: Nearly one-half to three-fourths of the surface marked, often 
with misshapen fruit. 

At picking time 20 loose, or " lug," boxes of oranges from the 

sprayed trees and 20 from an adjoining block of unsprayed trees 

were counted. The results obtained are given in Table II. 

• 
Table II. — Injury to sprayed <ui<l unsprayed fruit by orange thrips. 

SPRAYED. 



Num- 
ber of 
loose 

boxes. 


Total 
number 
of oranges 
exam- 
ined. 


Num- 
ber 
sound. 


Number 
slightly 
marked. 


Number 
moder- 
ately 
marked. 


Number 
badly 

marked. 


Per cent 

of sound 

fruit. 


Per cent 

of slightly 

marked 

fruit. 


Percent 
ofmoder- 

ately 

marked 

fruit. 


Per cent 

of badly 

marked 

fruit. 


20 


2,070 


1,533 


506 


31 




74.5 


24.5 


1 












UNSPRAYED. 



20 


2,365 


337 


1,047 


710 


271 


14. .3 


44.5 


30 


11 



A commercial grading of the sprayed fruit would have placed 
nearly 75 per cent as " Fancy " and the remainder as " Choice,'" while 
the unsprayed fruit would have run not more than 15 per cent 
"Fancy" and 50 per cent "Choice," the remainder going out as 
" Standards " and " Culls." Of the fruit counted from the unsprayed 
trees, 85.5 per cent was marked, while 25.5 per cent only of that from 
the sprayed trees showed injury, indicating that 60 per cent of the 
sound fruit was due to the spraying. The thrips-marked fruit was 
smaller than the sound fruit, as will be seen by comparing the total 
number of oranges from the 20 boxes of sprayed fruit with that from 
the 20 boxes of unsprayed fruit. The writers have frequently noticed 
in the packing houses that the smaller fruit is worse marked than the 
larger, making it appear that the thrips injury holds back the growth 
of the oranges. 



12 



INSECTS INJURIOUS TO SUBTROPICAL, FRUITS. 



The sprayed block contained 121 bearing trees. These yielded 165 
loose boxes of oranges. The unsprayed block contained 152 bearing 
trees, which yielded 162 loose boxes of oranges. The sprayed block, 
therefore, produced three more boxes of fruit, though containing 31 
less trees, than the unsprayed block. 

Experiment No. II. — A block of Washington Navel oranges em- 
bracing about 5 acres was selected in the spring of 1910 and treated 
three times with a spray of commercial lime-sulphur (33° Baume), 
1 part to 75 of water, combined with blackleaf tobacco extract. 1 to' 
150. A gasoline power sprayer was used and a pressure of 200 
pounds maintained. The first application was made May 4, the 
second May 17, and the third June 14. The first application was 
timed at a date when most of the petals had fallen. The second and 
third applications were made when the thrips became sufficiently 
numerous. An effort was made to keep the young fruit free from 
thrips until it was the size of an English walnut, as it appeared that 
this would insure a high percentage of clean fruit. 

Examinations and counts made of 92 loose boxes of sprayed fruit 
and 20 loose boxes of unsprayed fruit from an adjoining unsprayed 
" check " block gave the results shown in Table III. 

Table III. — Injury to sprayed and unsprayed fruit by orange thrips. 

SPRAYED. 



Num- 
ber of 
loose 
boxes. 


Total 

number 

of oranges 

exam- 
ined. 


Num- 
ber 
sound. 


Number 
slightly 
marked. 


Number 
moder- 
ately 
marked . 


Number 
badly 

marked. 


Per cent 

of sound 

fruit. 


Per cent 
of slightly 

marked 
fruit. 


Per cent 
of moder- 
ately 
marked 
fruit. 


Per cent 

of badly 

marked 

fruit. 


92 


8,458 


4,995 


3,383 


68 


12 


59 


39.9 


1 






UNSPRAYED. 



20 


1,G97 


498 1,108 


65 


26 


29.3 


05.2 3.8 


1.5 



The sprayed fruit shows a total of 40.9 per cent marked, while 71 
per cent of the unsprayed fruit was marked, and more severely. The 
amount of benefit due to the spraying was 30.1 per cent, which was 
considerably less than in 1909, due to the fact that the thrips were 
more numerous and infestation worse in 1909. The total output of 
oranges from Tulare County in 1910 was 50 per cent freer from 
thrips markings than in 1909. 

Experiments with Nursery Trees. 



SeYeral blocks of young nursery trees were sprayed in the fall of 
1909 with commercial lime-sulphur, 1 to 75, combined with black- 
leaf tobacco extract, 1 to 150. By two thorough applications it was 



THE ORANGE THRIPS. 13 

possible to save the tender foliage and axillary buds and to promote 
a growth of 1 to 2 feet more on the sprayed trees than on those 
unsprayed. 

Spkay Injury. 

While no actual spray injury, immediate or cumulative, developed 
from the use of distillate-oil emulsion and blackleaf tobacco extract 
at the strengths indicated above, the uncertainty of this combination 
as compared with the lime-sulphur and blackleaf tobacco extract 
led to the adoption of the latter as a spray for demonstration work 
during the season of 1910. 

RECOMMENDATIONS. 

In view of the success attained in reducing injury to fruit and 
foliage by the orange thrips, it is believed that it will be possible to 
control this species in normal seasons with four applications of lime- 
sulphur combined with blackleaf tobacco extract. 

TIME OF APPLICATION. 

Three of the treatments should be made in the spring to free the 
fruit and spring growths of foliage from injury, since the more 
severe marking of fruit is done while the fruit is small. The fourth 
treatment should be made in August or September, according to sea- 
son, for the protection of the later growths of foliage, and should be 
timed to catch the thrips when numerous, but before the leaves show 
much curling. The three spring applications should be made about 
as follows: 

First. Just after most of the petals have fallen from the blossoms. 

Second. Ten to fourteen days after the first. 

Third. From three to four weeks from the time of the second treatment. 

The dates for spraying in any given season must be timed by the 
abundance of thrips. 

SPRAY DILUTIONS. 

Lime-sulphur solutions should be diluted according to density. 
In the homemade product this may be determined by the use of a 
Baume or a specific gravity spindle. The density of the commercial 
product will be dated by the manufacturer or may be obtained by 
the use of the spindle. 

Lime-sulphur solution of a density of 33° Baume should be diluted 
at the rate of 1 volume to 75 volumes of water: that of a density of 
36° Baume should be diluted at the rate of 1 volume to 86 volumes 
of water. Therefore the formula for orange-thrips spraying 
would be : 



14 



IXSKCTS INJURIOUS TO SUBTROPICAL FRUITS. 



(1) Lime-sulphur (33° Baume) 1 to To and blackleaf tobacco ex- 
tract (2f per cent nicotine) 1 to 100; or, using blackleaf "40" (40 
per cent nicotine) tobacco extract 1 to 1,800. 

(2) If lime-sulphur of 36° Baume is used the formula would be. 
lime-sulphur 1 to 80 and blackleaf tobacco extract 1 to 100; or black- 
leaf tobacco extract "40" (40 per cent nicotine) 1 to 1,800. 

To load a sprayer having a 200-gallon tank, proceed as follows: 
First turn water into the tank until nearly full, add 2§ gallons lime- 
sulphur (33° Baume) and 2 gallons blackleaf tobacco extract (2f 
per cent nicotine) ; or 14 fluid ounces blackleaf " 40 " tobacco extract 
(40 per cent nicotine). If the lime-sulphur is 36° Baume. use 2.1 




Fig. 2. — Power spraying outfit in use in spraying for the orange thrips. (Original. » 

gallons, and 2 gallons of blackleaf tobacco extract; or 14 fluid ounces 
of blackleaf " 40 " tobacco extract. 

HOW TO SPRAY. 



In spraying for the orange thrips only those insects actually hit with 
the spray will be killed. As this insect obtains its food by sucking the 
plant juices, stomach poisons are of no avail. In order to spray with 
greatest efficiency it is necessary to use a gasoline power sprayer, 
maintaining a pressure of 180 to 200 pounds. (See fig. 2, showing 
a power outfit in operation.) Angles or elbows should be used on all 
spray rods so that " overshot " and " undershot " spraying can be 
done; that is, spraying from above downward, and from below up- 



THE ORANGE THRIPS. 



15 



ward to reach the lower surface of the leaves. The trees should be 
drenched until they drip freely. 

Especial care should be taken with the outside fruit as the thrips 
scar this badly, but cause little or no injury to inside fruit. 

Either chamber nozzles of the Cyclone type or Bordeaux nozzles 
may be used. If the former are used, disks with holes of about ^% 
inch diameter will be best. Double nozzles con be used to advantage 
on large trees, and will save time. It is preferable to use two lines 
of hose as this will insure more thorough work than where four leads 
are used. A majority of orange growers fail to apply a sufficient 
number of gallons of spray per tree. The following table will show 
approximately the correct amount to apply, and will enable those 
intending to spray to estimate the quantity of spray material needed 
for the season : 

Table IV. — Quantities of liquid required in spraying for orange thrips. 



Age of 

trees. 


One application. 


Total gal- 
lons of di- 
luted spray 
per acre of 
100 trees, 
4 applica- 
tions. 


Gallons 

diluted 

spray 

per tree. 


Gallons 

per acre 

of 100 

trees. 


Years. 
2-3 

5-7 

8-10. . . . 
12-14... 


2 
4 
5 
8 


200 
400 
500 
800 


800 
1,600 
2,000 
3,200 



SUMMARY. 

The orange thrips, a minute, orange-yellow insect of the order 
Thysanoptera, curls the leaves and scars the fruit of citrus trees in 
the San Joaquin Valley of California, the southern California orange 
belt, and the Salt River Valley of Arizona. 

Although this insect has been known by its work for some fifteen 
or sixteen years it has but recently been described, and it has now be- 
come of serious economic importance in the orange belt of the San 
Joaquin Valley of California. 

The orange thrips has numerous generations yearly, its life cycle 
requiring approximately 20 days, and it is to be found upon the 
orange trees from March to November. 

It can be controlled by four sprayings of lime-sulphur solution 
combined with a commercial tobacco extract, which should be applied 
when the thrips become sufficiently numerous. Three applications 
should be made in the spring months to save the fruit and spring 



16 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. 

growths from injury, and one in the fall to lessen the feeding injurj- 
to the fall growth of the orange trees. 

From two to eight gallons of this combination spray should be 
applied per tree, at a high pressure, and in a very thorough manner, 
as only thrips that are hit will be killed. 

Experiments in spraying have shown that three thorough applica- 
tions at the proper times have resulted in from 20 per cent to GO per 
cent more " fancy " fruit in the sprayed as compared with the un- 
sprayed blocks. 

O LIBRARY OF CONGRESS 




002 810 988 A 



